As above basically.
A lot of 4x4 vehicles used to employ selectable 4wd ie: a lever moved in the cabin which would select either high or low range 2wd, and/or high and low 4wd. Later models employ full-time 4wd which provides drive to either axle and utilises a centre diff which was lockable to get you out of the sticky stuff. The term full-time 4wd is a bit of a misnomer as without the centre diff locked, drive will always go to the point of least resistance ie: the spinning wheel. Locking the centre diff will therefore allow a wheel on the other axle to drive too, hopefully extracating you from your situation.
An improvement upon that would be the viscous coupling that does away with the need to shift to centre difflock as it is an automatic operation. Although I am guessing I would assume from your description that this is the system your vehicle employs.
Then there are axle difflocks, as fitted to Shoguns for instance. These work by locking the differential on a particular axle giving either wheel of that axle equal drive. Very effective although of limited use in the towing environment.
Most if not all new Land Rover models use the ABS system to keep moving in the gloop. By braking each wheel as it starts to spin a form of traction control is introduced, and very effective it is too. The only downside being that it isn't mechanical and therefore relies on electronics to operate. Not ideal when you are up to the windows in water.
Mind you, it's a sight to see a L322 Rangey spinning all four wheels at the same time as it climbs over a sodden wet log.
HTH
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